FORMATION OF THE AIMBULACEAL SYSTEM. 35 



mistaken for an encroachment on the stomach itself. In proportion as 

 the abactinal region becomes sohdified (PI. III. Fi<j. 11; PI. IV. Figs. 1, 

 2; PI. VII. Fig. 8), the stomach loses its globular shape, and becomes 

 from this time forward flattened and pear-shaped. Previously to the for- 

 mation of the Starfish on the surfoce of the two water-tubes, placed on 

 opposite sides of the stomach, we could trace no change of form in the 

 stomach itself From the time, however, when the Starfish encroaches 

 little by little upon the anal extremity of the larva^, it pushes the stom- 

 ach and the intestine slightly to one side, owing to the great increase in 

 bulk of its actinal and abactinal areas. The anal portion of the water- 

 tubes now swells and contracts in such a way as apparently to divide that 

 portion of the water-tubes into chambers ; but, on watching the circula- 

 tion of the fluid in the water-tubes for any length of time, the currents 

 can be followed flowing from one of these elliptic chambers to the other, 

 plainly showing the different planes in which the ventral and dorsal part 

 of the tubes are placed to be the only cause of this delusion. 



Miiller has distinctly stated, over and over again, during the course of 

 his investigations, that the young Echinoderm was formed by encroaching 

 upon the stomach itself; I am satisfied, from repeated observations of this 

 point, in Starfish, Sea-urchin, and Ophiuran larvfe, that this is not the 

 case. The mistake arises from the fact that the water-tubes, by their 

 extension and increase, cover and conceal part of the stomach, forming a 

 sort of hood over it ; while the two sides of the young Echinoderm, 

 separated by the whole width of the stomach and the thickness of the 

 two water-tubes, form upon the outer surface of the latter, and do not in 

 an}' way encroach upon the stomach, which is simply enclosed by the 

 actinal ;ind abactinal areas of the Echinoderm. Had I not traced this 

 with the greatest care, I should scarcely venture to doubt the statements 

 of Miiller, but I am satisfied that he was mistaken in this explanation of 

 the mode of the formation of the Echinoderm.* 



Formation of the Ambulacral System. — We have already seen that the 

 very first changes which take place in the water-system {w, iv) consist 

 of the five folds (t, PL V. Fig. 2) extending obliquely across the exterior 

 surface of one of the water-tubes («''). From the fiict that these folds 



* It may not be out of place to sav, that Professor Agassiz, during this investigation, satisfied him- 

 self of the accuracy of every point which seemed in the least contradictory to the statements of 

 Miiller. 



